EPR study of an associative polymer in solution: determination of aggregation number and interactions with surfactants

Abstract

It has been well established that hydrophobically end-capped water-soluble polymers, such as poly(ethylene oxide) didodecyl ether, C₁₂EO₂₀₀C₁₂, self-associate in aqueous solutions by virtue of the hydrophobic effect and thus form hydrophobic domains comprising the end-groups. These hydrophobic domains are able to solubilize spin probes which are virtually insoluble in the aqueous phase, enabling them to be investigated by EPR. An estimate of the number of polymer end-groups participating in each domain can be obtained by studying the details of the changes in the fine structure of the EPR lineshape as a function of the average number of spin probes per domain. Using this technique, the aggregation number of C₁₂EO₂₀₀C₁₂ in a 2.5 wt.% solution has been estimated to be 31 ± 6 end-groups per hydrophobic domain. The microviscosity, (the local viscosity of these domains), estimated from the rotational correlation time of an aminoxyl spin probe was invariant with polymer concentration in the range 0.25–5 wt.%. The micropolarity, or the ‘effective relative permittivity', estimated from the nitrogen hyperfine coupling constant, was also invariant in this concentration range. Therefore, the structure of the hydrophobic domains remained similar as the polymer concentration was increased, while the macroscopic viscosity was known to increase dramatically in this concentration region. A model which incorporate these observations is discussed.

The hydrophobicity and microviscosity of the hydrophobic domains are also compared and contrasted with values for surfactant micelles. Interactions of surfactants with the polymer have been monitored by measuring the spin-probe sensed polarity, as a function of surfactant concentration, at two concentrations of polymer. At ratios <3 sodium dodecyl sulfate (SDS) molecules per end-group, the polarity increased from that of the hydrophobic domain towards that of the anionic SDS micelle. On addition of the non-ionic surfactant C₁₂E₂₃, the polarity decreased from that of the hydrophobic domain toward that of the neutral C₁₂E₂₃ micelle. Addition of both types of surfactants therefore altered the composition and thus the properties of the hydrophobic domains. At high surfactant concentration, the evalution of the spin-probe sensed polarity is more complicated.